Conventionally, three-phase machines are operated with the help of pulse-controlled inverters. The three-phase machines are connectable to a battery through the pulse-controlled inverter. The pulse-controlled inverter includes circuit breakers, which are usually implemented as a bridge configuration having six self-locking switches. For example, such self-locking circuit breakers may be power MOSFETs, but it is also known to use other semiconductor switches, such as IGBTs or thyristor-type switches, for example MCTs, ESTs, GTOs, or similar. Embodiments having traditional bipolar power transistors are also known.
All of these switches may only be used when the voltage at the control electrode exceeds a certain positive value compared to a reference electrode, for example the source electrode or the cathode. Without control voltage, the switches used in the conventional systems cut off. Actuating the switches, i.e., feeding a voltage to the control electrodes, is usually done with the help of a control device which is for example a component of a vehicle electrical system control unit or of a voltage controller.
If the three-phase machine is a permanent-field synchronous machine, in the event of an error condition there must be assurance that it is possible to short circuit the phases of the machine or connect them with low impedance, since otherwise overvoltages may occur when the synchronous machine is running at correspondingly high speeds. It is therefore conventional, and is described, for example, in German Patent Application No. DE 198 35 576, that in the case of a three-phase machine having a pulse-controlled inverter that is designed as a permanent-field synchronous machine, in special cases depending on the error condition, by switching on single or multiple circuit breakers selectively the phases of the machine are short-circuited, so that overvoltages are prevented.
However, actuating the circuit breakers in the conventional embodiment is only possible if correct functioning of the switches, the actuating system, and the battery is ensured. If individual switches of the pulse-controlled inverter are closed and yet the supply voltage has simultaneously failed, it is no longer possible to actuate the switches and turn them on.